1. Field of the Invention
The present invention relates to an aberration correcting element and an aberration correcting assembly for correcting aberration of a light beam.
2. Description of the Related Art
Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) are well known as information recording media for optically recording and reproducing information. Furthermore, there are various types of optical discs, for example, optical discs for reproduction only, write-once optical discs on which only additional recording can be done, and rewritable optical discs on which information can be erased and re-recorded.
Research and development are in progress for developing high-density optical discs and optical pickups and information recording and/or reproducing apparatuses (hereinafter referred to as recording/reproducing apparatus) applicable to the high-density optical discs. In addition, research and development have also been pursued for developing optical pickups and information recording/reproducing apparatuses having the capability to be used for optical discs of different types.
A method of using with the high-density discs by increasing a numerical aperture (NA) of an objective lens provided in the pickup apparatus has been considered. Another method is the use of a light beam having a shorter wavelength.
However, the aberration of the light beam caused by an optical disc is increased as the numerical aperture NA of the objective lens is increased or a light beam having a shorter wavelength is used. This makes it difficult to improve performance accuracy of the recording/reproduction of information.
For example, when an objective lens having a large numerical aperture is used, the amount of birefringence distribution, which depends on the incidence angle, is increased at the pupil surface of the optical disc, since the range of the incidence angle of the light beam to the optical disc is increased. This creates an aberration problem due to the birefringence becoming more influential. In addition, the effect of aberration produced by error in the thickness of a disc cover layer for protecting the disc recording surface increases.
One example of such an aberration correcting element is disclosed, for example, in Japanese Patent Application Kokai H10-269611. The aberration correcting element has a plurality of coaxial electrodes made of transparent material. The orientational ordering in a liquid crystal is adjusted by changing the voltage applied to each of the electrodes. Thus, the aberration produced in the light beam is corrected.
It is necessary to align the aberration correcting element precisely with the objective lens in order to correct the aberration accurately. Accordingly, in some cases, the aberration correcting element is previously aligned with the objective lens and fixed to the lens to form an integrated aberration correcting assembly. The aberration correcting element is aligned with the objective lens by adjusting relative positions while applying a voltage to the transparent electrodes of the aberration correcting element to produce diffraction and observing an image through the objective lens with an imaging device such as a CCD camera when manufacturing the aberration correcting assembly.
It is a problem in the above-described alignment method that the contrast of the transparent electrode pattern is unclear and, therefore, the alignment accuracy is low. Furthermore, it is difficult to ascertain the amount of positional deviation between the objective lens and the aberration correcting element when the positional deviation occurs.
The present invention has been made to overcome the above-described problems, and it is an object of the present invention to provide an aberration correcting element capable of being accurately aligned with an optical element, as well as a high-performance aberration correcting assembly in which an optical element and an aberration correcting element are aligned with high accuracy.
To achieve the object, according to one aspect of the present invention, there is provided an aberration correcting element which includes a first electrode layer having a plurality of electrode portions, a second electrode layer opposed to the first electrode layer, and a liquid crystal sandwiched between the first and second electrode layers for producing a phase change corresponding to voltages applied to the first and second electrode layers in a light beam passing therethrough, wherein the first electrode layer includes a plurality of position markers.
According to another aspect of the present invention, there is provided an aberration correcting assembly, which comprises a first electrode layer including a plurality of electrode portions and first position markers; a second electrode layer opposed to the first electrode layer; a liquid crystal sandwiched between the first and second electrode layers for producing a phase change in a light beam passing therethrough corresponding to voltages applied to the first and second electrode layers; and an optical element including second position markers corresponding to the first position markers.